It is the purpose of the present invention to produce an intrinsically accurate and inexpensive fuel-air metering device for internal combustion engines. The necessity for accurate fuel-air metering to I.C. engines is well understood by those skilled in the art. As pressures to improve engine efficiency increase and particularly as pressures to reduce exhaust emissions become more intense, the requirements for metering systems have become more stringent. At present, these stringent requirements are forcing fuel-air metering systems to become more and more expensive, and are also involving increased maintenance problems. It is extremely difficult to reproducibly program carburetors with the accuracy required, and carburetors also have problems with lags and with pulsating fuel flows due to air bleeds. Fuel injection systems of one sort or another are replacing carburetors in many applications, but these units tend to be expensive. One virtue of fuel injection systems is improved distribution of fuel from cylinder to cylinder. However, one of the inventors has, with his colleagues Kenneth Kriesel and Charles Siewert, invented a mixing vortex system with essentially perfect cylinder to cylinder distribution. This mixer eliminates the distribution advantage of multiple fuel point introduction. The present invention was worked out to replace a conventional carburetor upstream of this vortex mixing device, and is designed from the first principles of the fluid dynamics governing the fuel and air flow to produce accurate and programmable fuel-air metering in an inexpensive way.
An important objective of the inventors was to work out a design which could be made to function precisely, and with the function of the system in very close agreement with precise mathematical formulas. A system which can be modelled precisely by straightforward mathematical formulas has significant practical advantages, in that it requires less development, permits rational design changes in the system to be made, and permits the system to be straightforwardly programmed according to specified requirements.
It is important to emphasize that all fuel-air metering systems must function on the basis of the fundamental laws of fluid mechanics. As a minimum, any fuel-air metering device which is not a positive displacement device will have fuel governed by the incompressible flow equation (Bernoulli's equation) and will have the air flow governed by the compressible flow equation. These equations are exact in the same physical sense that the basic equations of Newtonian physics are exact, and in the same sense that the tabulated thermodynamic functions (for instance, entropy, enthalpy, and internal energy) are exact functions. In real systems, the mathematical equations governing a physical event are never true to perfect exactness because of unavoidable errors in shape or measurement, and because of physical effects which complicate the equations excessively. With the flow passage shapes typically used in prior art carburetors the flow behavior of the passages generally differs by so much from the basic flow equations that the basic equations have had limited practical value. Consequently carburetor and other fuel-air metering devices have evolved on an empirical basis. However, there are flow passage shapes which do in fact follow simple mathematical flow relations with excellent accuracy; if proper care is given to geometrical shapes the difference between mathematically predicted flow and real flow may be too small to easily measure. The details which must be tended to to produce this close correspondence between theory and reality are somewhat complicated, and explanation of these details form a significant part of this application.
One of the very important objectives in designing the present invention fuel-air metering system was to produce structures where the errors in the flow equations were extremely small and exactly calculable, so that the system would obey the flow equations to an extremely good level of approximation. By taking pains with the structures to see that the fluid mechanical equations are in fact met to excellent approximation, it is possible to have a system which can be predicted and designed reliably on the basis of precise and straightforward mathematics. The present invention metering system involves only the compressible flow equation for air flows, the incompressible flow equation governing the fuel flow, and simple geometry. It is therefore a fundamentally simpler system than that involved with injection systems using solenoid valves, and also a much simpler system than conventional carburetors which have a multiplicity of interlocking air-fuel control systems which interact in complex and analytically intractable ways.
In addition to the more mathematical aspects of the metering system design, the inventors have considered a number of practical economic and structural issues. For example, the system is designed to be compatible with inexpensive low pressure diaphragm fuel pumps, although it is also compatible with higher fuel pressure systems. Any system designed to meter to high accuracy must have parts built to a similarly high level of accuracy, but the inventors have taken pains to make sure that the parts of the system which must be made to close tolerances can be made so by simple manufacturing techniques. In addition, issues of durability as well as dynamic response have been considered.
Another issue of importance is the flexibility of the system with respect to different control strategies. The present invention is adapted to easily connect with either conventional control via diaphragms or with electronic air/fuel ratio controls of one sort or another. The interaction of the metering system with its control system is in each case analytically clear and straightforward. In addition, the parts involved in the control system can be made with relatively large absolute dimensions, so that they can be made to high relative accuracies.
It is the purpose of this disclosure to teach one of ordinary skill in automotive engineering to make and use the current invention fuel-air metering system. With this end in mind, the mathematical relations involved in the metering system have been set out formally and in considerable detail. Moreover, specific design issues relevant to the accuracy of the fuel-air metering system in practice are addressed.
The aforedescribed objects and advantages will become more apparent when taken in conjunction with the following detailed description and drawings illustrating by way of example preferred embodiments of this invention.